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the same time being able to precisely measure forward
produced particles in a wide range of transverse
momenta. In this way, the entire range of the saturation
region indicated in Figure 13 would become accessible.
While the present design of the ALICE experiment
is perfectly suited for the thorough characterisation of
particle production in the central region, the addition
of instrumentation in the forward rapidity region will be
indispensable for the study of the saturation region. This
could be achieved with a state-of-the-art high-granularity
electromagnetic calorimeter currently under evaluation
as a possible upgrade of the experiment.
Traditionally, in high-energy physics, hadron accelerators
pave the way to discoveries while the electromagnetic
probes provide precision tools. Therefore, in the long-
term perspective, electron-proton and electron-ion
collision data will be indispensable for a precise and
unambiguous characterisation of the small-x structure
of nuclear matter and the saturation regime. The currently
available data from e+p scattering at HERA have
been shown to be consistent with the gluon saturation
picture, but can also be explained by linear evolution.
Stronger signals of gluon saturation from DIS will require
e+p or e+A collisions at still higher energy compared to
HERA. A future high-energy hadron-electron-collider
(LHeC), as currently being discussed as a future project
at CERN, would be designed to penetrate deeply into the
saturated region, thus providing a unique opportunity
for determining the saturation scale and characterising
the properties of the saturation region.
Box 5. Gluon Saturation
While ordinary substances exist as gases, liquids or
solids, there are states of matter that evade this classification.
In particular, glasses appear as solids on short
time scales, but actually flow like liquids over much
longer times. A state of such ambiguous properties is
predicted to be visible in high-energy collisions.
It has been known for a long time that the quarks
inside nucleons and nuclei are ‘glued’ together by
so-called gluons, the force-carriers of the strong interactions.
The actually observable constituents of, e.g., a
proton depend on the resolution used as illustrated in
the figure. For coarse spatial resolution (i.e., low energy,
left side of the figure) one observes mainly the three
valence quarks which, e.g., comprise the total charge
of the proton. When increasing the beam energy, the
spatial resolution is enhanced and one can observe
more and more colour charges (mainly gluons, but
also quarks and antiquarks). These particles carry ever
smaller fractions x of the total momentum of the proton.
At very high energy (right side), the density of gluons is
so large, that they are no longer seen as independent
particles, but form a new state of matter, the classical
field limit of the strong interaction. The density of gluons
in this saturated state is high enough for the colour
field to exhibit classical properties. At short time scales
relevant for particle production, the state appears to be
frozen as in a solid. Over long time scales, however, it
evolves slowly, like a glass.
The enormous increase of the gluon number with
small x is understood from splitting processes of gluons.
At high enough density, however, gluons will collide and
merge frequently enough to lead to a balance between
splitting and merging. This will lead to a saturation density.
It is characterised by a characteristic maximum
momentum, the saturation scale Qs, which can be
Quark Gluon structure of a nucleus seen at increasing collision
energies.
calculated theoretically. In nuclei, the projected area
densities of gluons should be even higher and effects
of gluon saturation should thus be stronger, leading,
e.g., to a larger saturation scale.
Our understanding of interactions in microscopic systems
relies on the existence of quanta (like the photon
for the electromagnetic interaction), but in macroscopic
physics interactions show the properties of classical
fields. So far, the electromagnetic interaction is the only
example where we observe both manifestations. On the
one hand, gravitation has a clear classical phenomenology,
but the description (and observation) of its quantum
nature is one of the big puzzles in physics. Subatomic
interactions, on the other hand, are genuinely quantised
– we observe the quanta in particle physics experiments,
but no classical system has been observed
yet, where the individual quanta would be no longer
important. Of those interactions, the weak interaction
offers no hope to study this effect experimentally. For
the strong interaction, the predicted new state of matter
can be explored experimentally in ultra-relativistic
electron-nucleus and proton-nucleus collisions.
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